Control means for dynamic braking



Dec. 26, 1950 A. C(DYER CONTROL MEANS FOR DYNAMIC BRAKING 3 Sheets-Sheet1 Original Filed July 9, 1947 Mi I J4 Zla 22a -o| 0-41.-

INVENTOR. ALVl/V C DYE)? BY m f ar'roznreysQ Dec. 26, 1950 A. c. DYE-RCONTROL MEANS FOR mum-11c BRAKING 3 Sheets-Sheet 2 Original Filed July9. 1947 INVENTOR. ALVIN c. DYE)? Dec. 26, 1950 A. c. DYER 23,312

CONTROL MEANS FOR DYNAMIC BRAKING Original Filed July e, 1947 aSheets-Sheet 3 9770 always parallel connected series motors which matlcmeans for efiecting for each motor when braking "eight contacts dynamicbraking circuits.

'larly important then the dynamic braking contacts are prefer- -tured,are single pole contactors.

Reissued Dec. 26, 1950 CONTROL MEANS FOR DYNAMIC BRAKING Alvin C. Dyer,Shaker Heights, Ohio, assignor to The Electric Controller &Manufacturing Co., Cleveland, Ohio, a corporation of Ohio Original No.2,497,492, dated February 14, 1950,

Serial No. 759,817, July 9, 1947.

Application for reissue April 12, 1950, Serial No. 155,541

21 Claims.

Matter enclosed in heavy brackets reissue specification; matter printedin italics This invention relates to control systems for for dynamicbraking automatically regardless of the direction of rotation of themotors, and more particularly to a motor control system which permitsoperation of less than all of a group of series motors while maintainingoperative autodynamic braking of the motors being operated.

If a separate controller is used to control each motor of a group ofseries motors for operation in parallel with each other, four contactsare required to complete the dynamic braking circuits is to be producedby all of the motors from either direction of rotation. Consequently,when two separate controllers are used to control two series motors foroperation in parallel with each other, a total of [are] is required tocomplete the close to complete the braking circuits after forwardoperation of the motors and the other four close to complete the brakingcircuits after reverse operation of the motors.

For reasons of economy, simplicity, and spacesaving it is desirable touse as few dynamic braking contacts as possible whether of themechanical or electronic type. This is particufor automatic operationbecause ably contacts of spring-closing electromagnet contactors which,as most commonly manufac- Consequently, a material reduction in thenumber of dynamic braking contacts results in a much smaller andconsiderably less expensive controller.

Control systems are known, for example such as described in North PatentNo. 1,699,743, for dynamically braking a pair of direct current seriesmotors by completing two closed loops each including the field Windingof one motor and the armature winding of the other motor. As a result ofsuch interconnection of the two mo- .tors, the number of contactsrequired to complete 'the dynamic braking circuits for braking of the-motors after operation in either direction is reduced from the usualeight to but four. If an 'even number of motors greater than two are to:be operated in parallel. with each other, the rhotors may be grouped inpairs to permit intercon .nection of the motors of each pair for brakingpurposes. required for each pair of motors so that, it 'four motors areused, a total ingcontacts is required. The controller of the Fourdynamic braking contacts are of eight dynamic brakprovide V Four ofthese contacts appears in the original patent but forms no part of thisindicates the additions made by reissue present invention may be used tocontrol two or more motors and requires a maximum of only four contactsto complete the dynamic braking circuits for braking from eitherdirection of rotation.

A further disadvantage of dynamic braking control systems .in which themotors are so interconnected that one motor supplies-excitation currentfor the other is that in the event of failure and consequentdisconnection of one of the two motors of a pair, the remaining motorwhen operated alonecannot be stopped by dynamic braking. In manyapplications requiring the use of two or more series motors, such as forexample the bridge drive of traveling cranes, it is desirable to haveavailable dynamic braking action even though one or more of the motorsis disconnected. The motors of the present invention are so connectedthat each motor supplies its own excitation during dynamic brakingthereby permitting one or more of the motors to be disconnected withoutdisturbing the dynamic braking action of the remaining motors.

It is an object of this invention to provide an improved dynamic brakingcontrol system for two or more parallel connected direct current motorshaving series field windings.

Another object is to provide an improved con trol system for a pluralityof direct current motors having series field windings that is capable ofrendering all or less than all of the motors effective for dynamicbraking when operating in either direction.

Another obiect is to provide a control system for dynamically brakingtwo or more direct cur-- rent series motors from either direction ofrotation which requires onlv four contacts to complete the dynamicbraking circuits.

Another object is to provide an improved motor control system which isoperative regardless of the direction of motor rotation to complete adynamic braking circuit for one of two series motors if the other motoris disconnected.

Another object is to provide a motor control system for dynamicallybraking two or more direct current series motors from either d rectionof rotation which requires only four contacts to complete the dynamicbraking circuits and which permits disconnection of one or more of themotors without disturbing the dynamic braking circuits for the remainingmotors.

Another object is to provide an improved motor control system comprisingmeans for disconnccting 9.9? QI':mD1B,11'l0tO1S of a. group oi-P allelconnected direct current series motors arranged for dynamic brakingwithout disturbing the dynamic braking circuit of the unconnectedmotors.

Another object is to provide cross-connections between like potentialpoints in the circuits of two or more parallel connected direct currentseries motors which cross-connections permit a reduction in the numberof contacts required to connect the motors for dynamic braking fromeither direction of rotation.

A further object is to provide an improved means for limiting thedynamic brakingtorque'of a direct current motor.

A further object is to provide a dynamic brak ing control system for adirect current series mo-- tially the same potential, and which includesar mature cross-connections between the terminals of like polarity ofthe armature windings and field cross-connections between the terminalsof like polarity of the field winding which crossconnections areselectively interconnected with each other for braking by contactsrendered operative selectively depending upon the direction of motorrotation.

A controller built in accordance with this invention includes means forconnecting the several motors of a group of direct current" seriesmotors in parallel acro s a source of power with each motor in serieswith its own accelerating and plugging resistor. The motor connectionsare such that like polarity terminals of the several armature windin sare at substantially the same potential and that like. polarityterminals of the several. field windings are at substantially the samepotential. Cross-connections connect the like polarity terminals of thearmature and field windings to commonjunction points. respectively.These cross-connections become parts of the dynamic. braking circuitswhich are completed by connections between selected pairs of the commonjunction points of the several cross-connections. The properconnections, between the several cross-connections may be. sel ctedautomatically. in dependence upon. the direction" of rotation of themotors by a plurality of springclosing. contactors responsive to. thecounter volte age of one of the motors. Cut-out knife switches are.provided on the controller and are so arran ed that any motor may bedisconnected without disturbing the dynamic braking circuitsfor theremaining motors.

Other objects andadvantages' of this invention willbecome apparent from.the following descriptionwhereinreference is made to the drawings, inwhich.

Fig.1 diagrammatically shows apair of. motors connected to a commonload;

Fig. 2 is a wiring diagram illustratingthepower circuits-and some of thecontrol circuits of a nreferred embodiment of the invention arrangediorcontrolling the two motors of. Fi 1;

Fig. 3' is a wiring diagram showing the remaining. control. circuits ofthe preferred embodiment;

' Fig. 4 is a simplified wiring diagram, showing the power circuits thatare energized when both motors of Figs. 1 and 2 are operating as motors;

Figs. 5 and 6 are simplified wiring diagrams showing thev power circuitsthat are energized when both motors of Figs. 1 and 2 are being brakeddynamically from forward and reverse directions, respectively, and;

Fig. 7- is a simplified wiring diagram showing how Figs. 2 and 3 may bemodified for controlling more than. two motors.

Figs. 2 and3 when combined illustrate a com-- plete motor controlsystem. Some of the contactors and relays-of Fig. 2 are shownincompletely in.Fig. but'allcontactors and relays shown in Fig; 3are-shown completely in that figure, and

the contactors and relays that are shown incompl'etely in Fig. 2 areshown completely in Fig. 3.

As illustrated. in Figs. 1. to 6., a controlsystem in accordance withthis invention is arranged'to control a pair of reversible directcurrent series motors. ii]. and M (Fig. 1-) to be operated in parallelwith each: other for driving a common; load [-3. The motor in has anarmature-winding 10a (Figs. 1 and 2) and a series field winding [0b andthe motor H has an armature winding Ma and a-series field winding H b.Although the motors illustrated are series machines, it will beunderstood that the system can control compound motors as well.

Power may be supplied tothe'motors lfl and l I from the conductors l2and I! (Fig. 2). which are arranged to be connected by atwo-pole knifeswitch l5to a suitable source of power represented by the conductors l6,To permit selective opera;- tion of either motor alone" or. both' motorsto gether, suitable switching'means' such as knife switches I9 and 20are provided; The knife switch I9 is associated with the motor ii!vand-has poles lilato I'9g inclusive; the poles Nd and [9f beingdouble-throw. The knife switch 20' isassociated with the motor II' andhaspoles 20a to 211i, inclusive, the poles 20d, 20f, 20h, and 201? beingdouble-throw.

To permit the control" system to be arranged so that the motors I0 and lI may be: easily and automatically disconnectedfrom the'pow'err ourceupon a decrease in the supply'voltagezor upon an overload, a pluralityof el'ectromagneticcontactors 2!, 22, and 24 are provided ior'reversiblyconnecting the motors I0 and H inparall'elwith each other between theconductors l2 and. I 4. The contactor 2| has four normally open maincontacts 2'la, 2Ib, Ho, and Md and the'contactor 22 has four normallyopen main contactsfla', 22b, 22c, and Md. The contacts 21a and. 21b whenclosed connect the armature winding I [la for forward rotation of themotor Dandthecontacts 2 lo and 2 Id when closed connectthelarmarturewinding I la for forward rotation of the motor l I Similarly; thecontacts 22a and 22b when closed connect the armature winding Illafor'reverse rotation of the motor Ill' and the contacts 220 and 22d whenclosed connect the armature winding Ila for reverse rotation ofthemotorr'l l. Although, as shown in Fig. 2; the current-in the armaturewindings Illa and Nb is reversed to effect reversal of the motors; itwill be-apparent that the control system could also be arranged toreverse instead the current in thefield windings I 6b and I lb. Thecontactor 2'4 has a normally open main contact 2 5a which when closedcon- :nects the motors l0 and II to the conductor II. The contactors 2|,22, and 24 have operating h windings 2lw, 22w; and MW, respectively: As

shown ihFig; 3; the contactor?! has-normally open auxiliary contacts2le,2lf, and2lg, and likewise normally open auxiliary contacts 22e, 221, and22g are provided on the contactor 22. The contactor 24 has a normallyopen auxiliary con tact 24b.

Acceleration and plugging aswell as the speed of the motors l0 and Ilmay be controlled by suitable means such as the series resistors 25 and26, respectively. Plugging sections 25a and 26a of the resistors 25 and28, respectively, are arranged to be short circuited concurrently uponclosure of main contacts 28a and 28b, respectively, of anelectromagnetic plugging contactor 28 having an operating winding 28w.The contactor 28 has a normally open auxiliary contact 28c'and anormally closed auxiliary contact 28d. Accelerating sections 25b and 26bof the resistors 25 and 26, respectively, are arranged to be shortcircuited concurrently upon closure of main contacts 29a and 29b,respectively, of an electromagnetic accelerating contactor 29 having anoperating winding 29w and a normally closed auxiliary contact 29c.Additional accelerating resistor sections and contactors may be providedif desired.

Operation of the contactor 28 is controlled by a suitable plugging relay30 having a normally closed contact 30a and an operating winding 38wwhich is connected in parallel with the resistor section 26a. Operationof the contactor 29 is controlled by a suitable accelerating relay 3ihaving a normally closed contact 3ia and a series-type operating winding3|w which is connected in the short-circuiting loop completed by thecontact 28b. The relay 3| is preferably of the type described inTrofimov Patent No. 1,980,736 and has its contact 3la mounted on aconducting tube 3Ib that moves upwardly to open the contact 31a upon anincrease in current in the winding 3lw and returns to the normallyclosed position shown after a time interval. Other types of plugging andaccelerating relays than those shown may be used if desired.

The dynamic braking circuits to be described are controlled by suitablecontrol means which includes the contacts 32a, 32b, 34a, and 34b,hereinafter described. Thus, in the illustrative example, the dynamicbraking circuits are completed by selective closure of normally closedmain contacts 32a and 32b of an electromagnetic forward brakingcontactor 32 and normally closed main contacts 34a and 34b of anelectromagnetic reverse braking contactor 34. Any suitable means may beused to control the selective closure of the contactors 32 and 34 inaccordance with the direction of motor rotation, but preferably for thispurpose the contactors 32 and 34 are provided with polarizing windings32p and 34p,- respectively, as described and claimed in a copendingapplication of J. D. Leitch and P. G. White, :Ser. No. 736,146, filedMarch 21, 1947. The contactors 32 and 34 also have operating windings32w and 34w, respectively, normally open auxiliary contacts 320 and 32dand 34c and 34d, respectively, and normally closed auxiliary contacts32c and 34e, respectively. The windings 32p and 32w have a commonmagnetic circuit and thewindings 34p and 34w also have a common magneticcircuit as indicated. Although each of the contactors. 32 and 34 hasbeen shown as a double-pole contactor, this hasgbeen done merely tosimplify the drawing since usually two single-pole contactors would beused instead of one double-pole contactor.

A protective resistor 35 for the polarizing windings 32p and 34p isby-passed at slow motor speeds by a normally closed contact 36a of acounter-voltage relay 36 having an operating winding 36w, an additionalnormally closed contact 36b, and a normally open contact 36c. Aprotective resistor 38 for the windings 32w and 34w is by-passed beforeacceleration and during plugging by a normally open contact 39a of atime delay relay 39 having an operating winding 39w. Preferably therelay 39 is of the flux-decay type with the contact 392. being delayedin opening. Energization of the windings 32W and 34w is controlled by anormally open contact 40a of a braking control relay 43 having anadditional normally open contact 40b and an operating winding 40w. x

The dynamic braking circuits include crossconnections 4| and 42 betweenlike polarity terminals, respectively, of the armature windings Illa andHa and cross-connections 44 and 45 between like polarity terminalsrespectively, of the field windings [0b and H b. Resistors 45 and 48having respective center taps 45a and 48a are interposed in thecross-connections 4i and 42, respectively, and resistors 49 and 53 areinterposed in series with each other in the crossconnection 44.Connections to be completed selectively for dynamic braking extend fromthe center-tap 46a through the contact 34b to the cross-connection 45 ata junction point 45a and through the contact 32a to the cross-connection44 at a junction point 44a intermediate of the resistors 49 and 5!).Likewise dynamic braking connections extend from the center-tap 48athrough the contact 32b to the cross-connection 45 at the point 45a andthrough the contact 34a to the cross-connection 44 at the point 44a.

It is to be noted that the mid-tap 45a is a common junction point forconductors leading from the left-hand armature terminals, the mid-tap48a is a common junction point for conductors leading from theright-hand armature termi nals, and that junction points 44a and 45a arecommon to the like polarity terminals of the field windings ltlb and Nb,respectively. As shown in Fig. '7, more than two motors may becontrolled by providing similar common junction points for like polarityarmature and field terminals of all of the motors.

Portions 49a and 50a of the resistors 49 and V 50, respectively, arearranged to be short-circuited by normally closed contacts 51a and 5lb,respectively, of an electromagnetic contactor 5! having an operatingwinding 5Iw. Adjustable portions 49b and 58b of the resistors 49 and 50,respectively, are arranged to be short-circuited by normally closedcontacts 52a and 54a, respectively, of electromagnetic contactors 52 and54, respectively, which have respective series-type operating windings52w and 54w interposed in the cross-connection 44 on opposite sides ofthe junction point 44a.

The direction of rotation and the speed of the motors l9 and il may beselected by a reversing master switch 55 (Fig. 3)" having electricallyinterconnected contact segments 55a through 55], inclusive, which aremovable with respect to cooperating contact buttons 55n to 55t,inclusive. The master switch 55 has 'anoff position and three forwardand three reverse positions as indicated. A low voltage protection relay5% having a normally open contact 55a and an operating winding 55w isassociated with the master switch 55 and maybe made responsive in theusual rrianner to the operation of overload relays :(not'shown); Dynamicbraking 01 the motor-sit and H- may be. controlled by a master switch58' having electrically interconnected contact segments 58a, 58b,-and58c which are movable from an on position through two bra king positionswith respect to contact buttons 58d, 58c, and 58f, respectively.

A contactor 59 having an operating winding 59w has normally open maincontacts 59a and 59b which complete when closed connections throughresistors 60 and 6|, respectively, to provide slight excitation for thefield windings [b and Nb, respectively, while the motors are drifting"or: coasting. This slight excitation insures that the relay 36remains'picked-up and the windings 32p and 34p remain energized duringthe coastingperiod and further insures that the field windings of themotors build up if the motors are braked after coasting.

Further understanding of the'preferred construction and arrangement of'the component parts of the controller of Figs. 2 and 3"may be had fromthe following description of operation:

First it is assumed; that bothof the motors l'fl and H are to beoperated together in the forward direction and then stopped by dynamicbraking. With; the knife switch I5 closed to char-- 'gize the conductorsl2 and I 4, both motors are arranged for operation together when theknife switch [9 is in its upper closed position and the knife switch isin its lower closed position.

With power supplied to the conductors l2 and H and both master switches55 and'58 in their err positions as shown, energizing circuits (Fig. 3)are'completedfor the windings 39w, w, 51w, and 56w. The circuit for-thewinding 56w is from the conductor I! through the contacts 36b, 55h, 55a,55b, 550, a conductor 54, and the winding-Sfiw to the conductor l 4.'Energization of the winding 56w causes closure of the contact 56a tocomplete a'circuit from the conductor I; to the conductor 64 and thewinding 55w which is independent of the positionof'the master switch55'. The circuit for the winding 5lw is from the conductor 64' throughthe contacts 58d, 58a, 58b,

and 58e through the winding 51W to the conductor I 4. The contacts soand 5|]; upon energization of the winding 51w open and interrupt theshort-circuiting paths around the resistor sections 59a and a,respectivelylFig. 2). The circuit for the winding 39w is from theconductor 64 through the contacts 3% and 34a in parallel, and thecircuit for the winding 40w is from the conductor 64 through the masterswitch contacts 58d, 58a, 58c and 58f.

Energization of the winding 40w causes closure of the contacts lfla and40b. Closure of the cont'act'flfla completes a circuit for the windings32w and MW from the conductor 64 through the contact 39a which closed toby-pass the resistor 38 uponenergization of the winding 39w.Energization of the windings 32w and MW with the resistor 38 by-passedcauses positive opening of the contacts 32a, 32b, 34a and 34b in thedynamic braking circuits. The contacts 3% and He also open to deenergizethe winding 39w. After a short: time deay, the contact 39a opens toreinsert the resistor 38 in serieswith the windings 32w and MW, but thecontactors 32 and 34 'remain-in their energized positions. The contacts326. and 34d which closed'upon energization'of the windings 32w and 34wtogether with the now-closed contacts' lflb partially complete cit-'-cuits tor'the windings 2 iw-a'nd 22w: 5 I? I If the master switch is nowmovedto any of the'forward positions, energizing circuits from theconductor 61 to the conductor; l4 are completed for' the windings Zlw,24w, and 59w. The circuit for the winding 2 [W includes the contacts550, 515 b, 55d, 55q, 4flb[;], 32d-"and 34d, and the circuits for thewindings MW and 59w include the contacts 55 [c] 0, 55b, 55c, and 55p.The contacts 290 are also in the circuit for the winding 59w.

Energization of the winding'2'4w causes closure of the contact 24a whichconnects the motors l0 and II to the conductor l4 and causes closure ofthe contact 25b in an energizing circuit for the Winding 28W.' With thecontact 24a. Closed: closureof the contacts 59a, 59b, Zia, 2l b ,"2'lcand Zld upon ehergizati'oh of the windings 59w and 21w causes bothmotors t o erat in the forward direction with respective armature 'shuntcircuits of relatively high resistance' Theeontacts 2le, 21f, and Zlgalso close. The'eontact 2le insures that the windings 32w and am areenergized when the winding z'rw' is energized, and the contact 2| gpartially'completes theta: cuit for the winding 28w. Closure of theContact 2lf completes a circuit for the winding saw, but the response ofthe'felay 39 is withoutopei'ative effectat this time. i As soon as poweris supplied to the motor 1 I, the voltage drop 'ac'rossitsarmature'winding I la causes energization of the winding 32p through thecontacts 320 "and 36a and energizatibn ofthe winding 34p throughthecontacts 34c and 36a; The contacts 32c and'34c closed upon energiza'tion of the windings 32w and34 w. After the motorhas 'reaehedapredetermined low speed, the winding 36w which'is connected across thearma ture winding H a becomes sufiiciently energized by thecounter-voltage of the motor H to open thecontact 35a thereby to insertthe resisto'r35 in series with the windings 32p and 34p. Means may beprovidedin'a 'well known manner for pro tecting the winding 36w againstoverheating when subjected to the higher 'values of counter voltage topermit the relay 36 to have a're a'tively low pick-up current value.With the motors op-' erating in'the forward direction, the flu'xproducedby the winding 32p opposes that produced by the-winding 32w and the fluxproduced by the winding 34p assists that produced by the winding 34w.The flux 'producedby the winding 32w, however, is sufliciently in excessof that produced by the winding 32p even at'the maximum possible speedof the motor II that the contactor 32' remains in its picked-up orenergized position.

It is assumed that the motors are accelerating from'rest' so that, whenthe master switch 55' reaches the second forward'position, a circuit iscompleted without time delay for the winding 28w through the contacts55c, 55s, Zlg, 30a and 2416. Closure of the contacts 28a and 2810 uponenergization of the winding- 28w short-circuits the resistor sections25aand 26a, respectively, and" increases the voltage applied to themotorarma tures. 4 Completion of theshort-circuiting'loop including thecontact 28bcausesenergization ofthe winding MW of the relay 3i whichrespondsto open its contact 3la. After a time delay-interval dependentupon the amount of current flowing' through the winding'3lw during theinterval; the contact 3| a recloses. moved to the third forward positionor if it fire; viously had been moved theraa'circui't isfco pletedfor'the' winding 529w through the contacts. 55f, 55t, Ma and 28c.Response of the contac'toi If the master switch is new 2! to theenergization of its winding 29w causes closure of the contacts 29a and29b which exclude the remainder of the resistors 25 and 26 from themotor circuits, and causes opening of the contact 29c to deenergize thewinding 59w. The contacts 59a and 59b thereupon open to interrupt thearmature shunt circuits through the resistors 60 and SI.

The motors Ill and l l are now operating in the forward direction attheir maximum speed for a given load and the motor circuits are as shownin Fig. 4. When so operating, the resistors 48, 49, and 50 serve tominimize any circulating currents in the cross-connections 42 and 44resulting from possible differences in voltage drops across thearmatures Illa and Ha. When the motors are reversed, the resistors 46,49 and 50 operate to minimize such currents in the cross connections 4|and 44. Only the portions 49b and 50b of the resistors 49 and 56 aresh-ort-circuited during normal running of the motors. If care is takento make the resistances of the two field circuits substantially equal toeach other, the currents in the field windings lllb and llb aresubstantially equal, and the load is satisfactorily distributed betweenthe two motors.

If at any time while the motors are operating in the forward direction,it is desired to effect a brakin operation, the master switch 58 may bemoved to its first braking position. This effects deenergization of thewinding 40w and consequent opening of the contacts Mia and 40b. Openingof the contact 40b deenergizes the winding 2lw and the contactor 2|returns to its normal position. Opening of the contacts 45a and Medeenergizes the windings 32w and 34w. The contactor 34 remains in itsenergized or pickedup position due to the flux produced by the winding34p. Since the flux produced by the winding 32p is opposed to thatproduced by the winding 32w before deenergization of the latter winding,the flux in the contactor 32 reaches zero and the contactor 32 dropsout. Opening of the contact 32c prevents further energization of thewinding 32p and increases the voltage applied to the winding 34p.

Closure of the contacts 32a and 32b upon dropout of the contactor 32completes the following dynamic braking circuit for the motor HI: fromthe left-hand terminal of the armature winding Illa, the knife switchpole l9b, the upper portion of the resistor 46 to the mid-tap 46a, thecontact 322., the point 44a on the cross-connection 44, the winding 52w,the contact 52a, a portion of the resistor 49, the knife switch poles Weand let, the field winding [b, the knife switch pole mg, the point 45aon the cross-connection 45, the contact 32b, the mid-tap 48a, the upperportion of the resistor 48, and the knife switch pole l9c to theright-hand terminal of the armature winding Ina. The following dynamicbraking circuit for the motor I l is also completed: from the lefthandterminal of the armature winding Ila, the knife switch pole b, the lowerportion of the resistor 45 to the mid-tap 46a, the contact 32a, thepoint 44a on the cross-connection 44, a portion of the resistor 55, thecontact 54a, the winding 54w, the knife switch poles 20c and 29f, thefield winding Hb, the knife switch pole g, the point 45a on thecross-connection 45, the contact 32b, the lower portion of the resistor48, and the knife switch pole 20c to the right-hand terminal of thearmature winding Ila. The motors are now connected as shown in Fig. 5.

Since the braking circuit for the motor l0 contains no part of the motorI l and the braking circuit for the motor ll contains no part of themotor If], an open circuit in one of the motors does not interfere withproper braking of the other motor.

With the master switch 58 in the first position, the contacts 5Ia and5lb are open and the resistor sections 49a and 55a are efiective in therespective dynamic braking circuits. As soon as dynamic braking currentflows in the cross-connection 44, the contacts 52a and 54a open due tocurrent flowing through the windings 52w and 54w. When the contacts 52aand 54a open, the resistor sections 4% and 55b become eifective in thedynamic braking circuit. The resulting decrease in the dynamic brakingcurrent causes the contacts 52a and 54a to reclose. Reclosing of thecontacts 52a and 542. results in an increase in current and the contactsreopen. The contacts 52a and 54a thus open and close in rapid successionuntil a low speed of the motors l0 and H is reached at which time thedynamic braking current is too low to cause further operation of thecontactors 52 and 54. Final stoppin of the motors l0 and II is effectedwith the dynamic braking circuits including only the resistors 45 and 45and the portions of the resistors 49 and 50 not bypassed by the.contacts 52a and 54a.

The contactors 52 and 54 are preferably designed with pick-up anddrop-out current values so related to the resistance of the resistorportions 49a and 55a and the expected value of dynamic braking currentthat each contactor opens and closes several times during dynamicbraking. By making the contactors 52 and 54 very rapid in operation, itis possible to limit the dynamic braking current to a predeterminedvalue at the start of braking and to maintain the average value of thecurrent but slightly below the predetermined value throughout a majorportion of the braking cycle.

When the master switch 58 is moved to the second braking position, thewinding 51w is deenergized and the contacts 5la and 5lb close. Strongerbraking is thereby produced since the resistor sections 49a and 50a areexcluded from the braking circuits. The contacts 52a and 54b open andclose repeatedly as before to graduate the braking current and torqueduring the stopping interval.

When the motors IB and II reach a predetermined low speed, the contact36a. of the relay 36 closes and short-circuits the resistor 35 so as toconnect the winding 34p directly across the armature winding i la. Thusthe contactor 34 remains in its energized position until a very lowmotor speed is reached. Preferably the contactors 32 and 34 have lowdrop-out current values so that the dynamic braking circuits remaincompleted as long as possible.

If the master switch 55 is moved to the reverse positions instead of theforward positions, operation during acceleration is the same as abovedescribed except that the winding 22w instead of the winding 2lw isenergized. The circuit for the winding 22w includes. the contacts 55h,55r, 45b, 32d, and 34d. Due to the reversal of counter- E. M. F. of themotors it and H for reverse operation, the flux produced by the winding32p now assists that produced by the winding 32w whereas the fluxproduced by the winding 34p during reverse operation opposes but is lessthan that produced by the winding 34w. Consequently when the windings32w and 34w are deenergized,

it the-eontactor 3% remains in its pickedeup. post. tions dueto the.flux produced. by thewinding 32p and the conta-ctor 34: drops out.because its .fluxis reduced to: zero. The contactors. 51, 52,. and. 54

respond during reverse braking in the manner described. abovefor-forward braking.

The-reverse braking. circuit for the motor L is through the upperportionof the; resistor; .48.

to the-mid-tap 48a, thecontact 34a, the. point 44a, the winding- 52w,theresistonfl, the fieldwinding Iflb, the point a, the contact 34b, andthe upper portionof the resistor-45 tothe. leftehand ter-. min'al -ofthe armature winding. 1.9a. The. reverse brakingcircuitiorthe motor I.l.is'through .the. lower-portion of the resistor 4-8, thecontact-Ma, thepoint 44a, the resiston thewinding 54w, the field winding Mb, thepointMia, the contact 34b,-:and: the lower portionof. the; resistor-.45;to

the lefthandterminal:= of the armature.v winding lia. "l hemotorsare nowconnected as shown in I f-the'motors I0 and: H: should bepl-ugged fromeither directionof rotation, the. relay. 38". opens its contactsZ-ma'rto prevent energization of the contactor 2-3 until themotorsapproach standstill; Acceleration-then-proceeds as before. Whenthe motor-1 t is plugged; the voltage across the. armature He becomesgreater-thanthe voltage.

between the conductors l2 and It. To prevent the-flux of the. windings342pv and- 34p from ex.- ceeding that produced by the-windingsu'izw and3ii'w-dur-ing--plug ging, the winding 39w of. the relay 39 1s energizedwhen the contacts2 ifsorZZ-f, andthe-contacts Z-Sdareclosed. Thecontact39a thus;closesduring-plugging to exclude the. 116:. sister 38fro'm;-th'e-- energizinge circuit for the. windings-- MW and BA-Wthereby increasing thefluxproducedby those-windings. so as. to maintainthe; necessary-excess-flux."

If it s desired tor-any reason to disconnect the motorl-iiiiromthecircum,theitnife switch 59 may be moved from its upperclosed-position to its loiveneldsedposition. With the-knifeswitch l9in-its-lowerclosed position, the poles [Sid and l9fconnectthe---resistorv 25's inpara llel: Withthe resistor 26. Thisincreases thetorque exerted by the; IliOtOP-lfl -Whll themasten switch is in thefirsttwo positions which; is desirable; since the motor l-l' -is now-drivingalone thesame-load I101:- mall'ydriven b'y bothmotors IIi'CtOI l9has-not altered the dynamic braking circu'it ior the-motor- H andthenioton H may be stopped hy dynamic b ralging in the same manner asdeserilid for 1 two-motor. operation.

= If the knife switch 2 9 is moved to itsupper closedp'o'sition'wliilethe kniie switch I '9 is in its upperclosedposition themotor H- isdisconnected from the circuitandtheresistors 25and25 are in parallelwitheach otherthrough the knife switch poles 29d andZGf and bothresistors are inlseries with the moton Hi.-- Thus the relays- 30: and 3|can-function-toc'ontrolplugging and acceleration of the motorlii; alone.

windings; 31w; 32p", and 34p across the armature ma throug l-i the knifeswitchpoles 2th and Zfii and does not alter the dynamic braking circuitof the motor [0.

If the knife switch poles We and 20eareoini tted; the resistors Miami50- would be in parallel during braking if one of the motors weredisconnected; from the circuit The resulting stronger braking torque forthe single remaining motor may be desirable for some applications.

Asis clearfrom- Figs, 2, 5, and 6, opening of the circuit: to the 7Moving of the knife switch 20 to itsgup'per closed'positionconnectsthewhen OlOSfidlQQDHEQtSl the motorsto-lthe conduc;

" Fig. 7 respectively; areprovidedand .rcpewer. shculdifailatany.=.time.during: 1; tion; o't either one or of. thBimOIlOIZS; therelaw56 opens its contacts. ifiaato deenergi-zethei con 3 ductor 64 With the.conductor. 64: deenergizedp alltof thelconta'ctors: and relays:- returnt.o...thei-r;- normal positionsnexcept the. relay. 3,6. and: the. properdynamic.brakingv contactor .321 or .3343 (16;; pending upon thedirection of motor rotat i n Braking eactionith-usl. proceeds. as if themaster switch 58.. had been-.movedto its. second position; During-.theshort interval between. the time; that, power istremoved': from: thearmature winding; .1; j; until: the. dynamic; bra-king. circuit ,for.the motor, 3' is established, the winding 3410; is energizedbythecounter-voltage resulting. fromthe-residu magnetism. in. the. fieldof: the..mo.t01';.-.l:l= fact that the contactsiatb are opendurin rakininsures that. power cannotxbe reapplied" to; the. motors after a. powerfailure until the, motors; reacha1ow.speed;....

. The circuitscthrcugh the contacts 59a: and: Eli-b ofsthe contactor.59:.and: the resistors; EB and 6 7 provide. a. small: excitation 01E thefield; winding;

meand- Mb respectively, while.the. motors-arev coasting, thatis,.w-hile. the..mot,ors, arerot t with power removed from thearmatnresan 9. dynamic. braking circuits interrupted. s sures. build-up.ot. the; motonfieldistrength; W, 3, brakingcohnec-tions. areestablished; and; insures; that the; relay 36'. and; thescontactors:32.; and 3 4 remain picked np while coasting. It; is be noted that sincethe field windings; lE-h, and-3U are. in parallel? during the build; up;of: onefield causes build up; of the; other. I 3 Fig. 7 shows. howv thecontrol system; of; Eigs 2; and :.3 may; be. modified; to control. more;than two. motors without; an increase in; the number-10ft;- dyi'n'a'micbraking-contacts; IntEig-i '7; a pl ur 'ty; ofi motors havingarmature windings; 'l la, lZ a, 13a and 14a. and respectivefieldwindingsfl b, 12h, 13b, and- Nb are. arranged; to beconn ted in;parallel. with. each. othen and; to.-.be. supplie irogn h a. suitable.power source indieated.by. thecon ductors 15zand': iiiz. normalix opencontact 18g tor 76, and a plurality of normally open tacte 19:. whenclosed: connectthe-several; armature windingsfortforwardoperationeithegnotorswhile; a plurality-.ofr normallyrvopen contacts841; controlreverse;operation.ofgthemotors .EhQQo ta'cts Til-8. 1.8;and. Blhmaybe .cbxitactso electroe.magnetic:contactorszand4corlicspondzto he v contacts.- [to]oyiwthetcontactors12:1; 22 and; 2 Fig. 2'. Acceleratingresistors. 8,1.for-themotors of; m

regulated in any suitable. manner.;

.The dynamic braking. conne'c-tione;w include; cross connections. 82and: 58; inclusive. 'I' cross. connection. 8:2; is. between the.left-hand terminalr ofr the. armature. winding 'igla,.,andy;the.left-handaterminalr.ofrthe armature winding Maandzth'e. cross;connection =8 5:;is;betwcen. the righthand .term inaistof these. two.armaturelwindinge Likewise,- thecross connections. M-andfllare bee.

tween opposite terminals. of: the iarmatu're wind?- ing-JZa must. Thecross. connection: 8&isb.e tween the left-hand: terminals of .the. field.win.-.. ings ll-b and: 14b andutht.crossconneetion.BJ-is; between-theleft-hand terminals of.- thelfield-winde ings 12b and. 13h Theright-handterminalspf; each of-the field windings. are-.interconnectedby the-crossconneotionhafl.

Resistors 89,- 90, 9|; aIIdtQZ areinterposedz-in. the cross connections82,83",v 84. and respec.-;. tively. Mid-tapsof theresistors 89 and Smare13 connected to a common junction paint and mid-taps of the resistors 9|and 92 are connected to a common junction point 95. Resistors 95 and 91are interposed in the cross connections 86 and 81, respectively, on oneside of a junction point 98 common to the cross connections 86 and 81,and resistors 99 and IE9 are interposed in the cross connections 86 and81, respectively, on the opposite side of the junction point 98. Theresistors 96 and 91 correspond'to the resistor 49 of Fig. 2 and theresistors 99 and I correspond toj'the resistor 50 of Fig. 2 and may beregulated in any suitable manner, but preferably in the manner disclosedin Fig. 2.

For controlling dynamic braking in dependence upon the direction ofmotor rotation, two normally-closed contacts [9| and two normally closedcontacts I02 are provided. The contacts llll connect the motors forbraking after a forward operation, and the contacts I02 connect themotors for braking after a reverse operation. The contacts I91 and H32correspond to the main contacts of the contactors 32 and 34 of Fig. 2,respectively. Means may be added to Fig, '7 in a manner hereinaboveexplained to disconnect any one or more of the motors while permittingthe remaining motors to operate.

' With the contacts [0| closed and the contacts I92 open the motors areconnected for dynamic braking if they have been running in the forwarddirection. Current flows from the armature Ha through the upper portionof the resistor 89, the junction point 94, one of the contacts lfll, theresistor 96, the field winding Hb, the crossc'onnection 88, theremaining contact Hll, the junction point 95, and the upper portion ofthe resistor 92 to the armature Ha. Current supplied from the armaturewinding Ila thus excites the field winding 1 lb to produce dynamicbraking action. The dynamic braking circuits for the remaining motorsmay be traced in a similar manner. If the motors are operating in thereverse direction the contacts IE2 are closed while the contacts ID! areopen and similar dynamic braking circuits for each of the motors arecompleted. It is thus seen that, irrespective of the number of motors,only four dynamic braking contacts are required in order to complete thenecessary dynamic braking circuits for braking from either direction ofmotor rotation.

' Thus, having described my invention, I claim:

1. A dynamic braking control system comprising a plurality ing anarmature winding and a series field winding, switching means forconnecting the windings of each motor in series with each other and saidmotors in parallel with each other across a source of power'foroperation thereof as series machines with terminals of like polarity ofsaid armature windings at substantially the same potential and terminalsof like polarity of said field windings at substantially the samepotential, said switching means including means for reversing saidmotors concurrently, armature cross connections connecting said armatureterminals of like polarity to common armature junction points,respectively, field cross connections connecting said field terminals oflike polarity to common field junction points, respectively, a firstpair of dynamic braking connections respective to said common fieldjunction points and independently connecting their associated fieldjunction points with one of said common armature junction points, asecond pair of dynamic braking connections respective to' said commonfield junction points and of direct current motors each hav- I 14independently connecting their associatedfiela junction points with theother one of said common armature junction points, and control means formaking and breaking said dynamic braking connections selectively.

2. A dynamic braking control system in accordance with claim 1characterized in that said control means includes contact meansinterposed in said dynamic braking connections, respectively, and isoperative to interrupt all of said dynamic braking connections when saidmotors are operating as motors.

3. A dynamic braking control system in accordance with claim 2characterized in that said control means includes operating meansoperative to complete a selected pair of said dynamic brakingconnections upon cessation of power supply to said motors whilemaintaining the remaining pair of said dynamic braking connectionsinterrupted, 5

4. A dynamic braking control system in accordance with claim 3characterized in that said operating means is operativ to close someofsaid contact means upon cessation of power supply to said motors whilemaintaining the remaining contact means open and includes meansresponsive to the direction of rotation of said motors to select which ofsaid contact means are to be closed and which are to remain open.

5. A dynamic braking control system in accordance with claim 1characterized in that means are provided for disconnecting one of saidmotors from the source of power independently of the operation of saidswitching means while maintaining said dynamic braking connectionsconnected with the windings of a remaining one of said motors through aportion of each of said armature cross connections, and a portion ofeach of said field cross-connections, thereby providing dynamic brakingloop circuits for said remaining one of the motors.

6. A dynamic braking control system in accordance with claim 1characterized in that said armature cross-connections define a closedarmature loop circuit that is completed at all times while said motorsare operating and which loop circuit includes two of said armaturewindings,

both of said armature junction points, and resistor means.

'7. A dynamic braking control system in accordance with claim 6characterized in that said field cross-connections define a closed fieldloop circuit that is completed at all times while said motors areoperating and which loop circuit includes two of said field windings,two of said field junction points, and resistor means.

8. A dynamic braking control system in accordance with claim 7characterized in that means are provided for varying said resistormeans.

9. A dynamic braking control system in accordance with claim 7characterized in that means are provided for short circuiting at least aportion of one of said resistor means, and comprise a contact normallyheld closed to by-pass a. portion of said one resistor means, a coil inseries with said one resistor means in the cross-connection containingsaid one resistor means between the common junction point and motorterminal of said cross-connection, said contact being responsive tocurrent flowing in said winding to open said short-circuit.

10. A dynamic braking control system in accordance wtih claim 1characterized in that resistors are interposed in said armaturecrossconnections between said common armature juncs connections, therebyproviding dynamic braking loop circuits for said remaining one of themotors.

ALVIN C. DYER.

REFERENCES CITED The following references are of record in the file ofthis patent or the original patent:

UNITED STATES PATENTS Number I Name Date 1,231,605 Hellmund July 3, 19171,699,748 North Jan. 22, 1929 1,985,706 Wilson et a1. Dec. 25, 19342,046,970 Royer July 7, 1936 2,128,034 Austin Aug, 23, 1938 2,248,577McNairy July 8, 1941

